Potassium is the most abundant intracelluar cation, and is very important in maintaining physiological homeostasis. Potassium channels are present in almost all vertebrate cells, and the potassium influx through these channels is indispensable for maintaining hyperpolarized resting membrane potential.
Large conductance calcium activated potassium channels (also BK channels or maxi-K channels) are expressed especially in neurons and smooth muscle cells. Because both of the increase of intracellular calcium concentration and membrane depolarization can activate maxi-K channels, maxi-K channels have been thought to play a pivotal role in regulating voltage-dependent calcium influx. Increase in the intracellular calcium concentration mediates many processes such as release of neurotransmitters, contraction of smooth muscles, cell growth and death, and the like. Actually, the opening of maxi-K channels causes strong membrane hyperpolarization, and inhibits these calcium-induced responses thereby. Accordingly, by inhibiting various depolarization-mediated physiological responses, a substance having an activity of opening maxi-K channels is expected to have potential for the treatment of diseases such as cerebral infarction, subarachnoid hemorrhage, pollakiuria, urinary incontinence, and the like.
There have been various reports on a large conductance calcium-activated potassium channel opener. For example, in International Publications WO96/40634 and WO99/36069, pyrrole derivatives have been disclosed, in Japanese Provisional Patent Publication No. 2000-351773, a furan derivative has been disclosed and in International Publication WO98/04135, a nitrogen-containing 5-membered derivative in which the nitrogen is substituted by phenyl group or benzyl group has been disclosed.